The traditional approach for design and manufacture of thin film electro-luminescent (TFEL) devices and displays is to incorporate zinc sulphide (ZnS) as the emissive phosphor layer, interposed between two dielectric layers and operated with electric fields of approximately 2 MV/cm, using an ac drive voltage with a frequency of approximately 1 kHz. Typically, the ZnS is activated optically with activator atoms, e.g. manganese (Mn) or rare earth atoms, to achieve intra-atomic transitions by impact excitation, thus creating light in the visible spectrum corresponding to the electron level configuration of the impurity atoms (Mn or rare earth). According to Y. Ono in a paper entitled “Electroluminescent Displays”, published in World Scientific, 1995, no known emission due directly to ZnS (bandgap of 3.6 eV, or 344 nm) has ever been reported in TFEL devices. Space charge separation during impact ionization is cited as the limiting factor in “Handbook of Electroluminescent Materials”, editor D. R. Vij, pp 36-37, Institute of Physics Publishing, 2004, hence the need to incorporate activator atoms.
Zinc oxide (ZnO) is a multifunctional semiconductor material which has been used in various areas, including phosphors, piezoelectric transducers, surface acoustic wave devices, gas sensors, and varistors. With a band gap of approximately 3.3 eV, ZnO is similar to that of Gallium Nitride (GaN), but with a higher free-exciton binding energy of 60 meV, compared to 25 meV for GaN, thereby favoring efficient free-exciton emission at room temperature. Free-excitons are coupled electron-hole pairs not bound to anything else other than themselves, i.e. they are perfect electric dipoles. In a semiconductor, they are equivalent to efficiently stored potential (light) energy, akin to a “light capacitor”. The high free-exciton binding energy in ZnO means that free-excitons can exist in ZnO at temperatures up to approximately 700° K., or 430° C., at which point they begin to “boil” apart and free-exciton recombination can no longer occur. Accordingly, ZnO is a promising material for light emitting devices that are both efficient and practical at room temperature. In comparison, the low free-exciton binding energy in GaN, i.e. 25 meV, results in the free-excitons “boiling” apart at or below room temperature, making GaN unsuitable for free-exciton light emission.
Another important property of ZnO is its high optical transmittance in the visible and near ultra-violet (UV) regions, even when it is doped with certain atoms, e.g. Aluminum (Al), which are used to increase the electrical conductivity of the zinc oxide film, thereby forming a transparent conducting oxide (TCO). Indium-tin oxide (ITO) is currently the industry standard for TCO material in flat panel displays, solar cells, etc; however, the global supply of indium metal is limited, thereby causing the price for the refined form of indium to be considerably higher than zinc, e.g. US$700/kg cf. for indium compared to US$4.00/kg for Zn, as of December 2006. Many leading electronics designers and manufacturers, e.g. Samsung, therefore have active development programs that aim to replace ITO with alternative TCO's, such as ZnO.
Zinc-oxide films have been synthesized by numerous methods, such as metal-organic chemical vapor deposition, molecular beam epitaxy, magnetron sputtering, pulsed laser deposition, atomic layer deposition, spray pyrolysis. Low temperature deposition is required in most flat-panel processes in order to avoid reactive and elemental diffusion of different layers and to protect substrates, such as polymers. Among these methods, ZnO films can be synthesized at temperature as low as 100° C. by metal-organic chemical vapor deposition and atomic layer deposition, and even at room temperature by magnetron sputtering and pulsed laser deposition. The high kinetic energies of growing precursors in the last two methods are believed to play a key role in the realization of low temperature deposition critical to the flat panel display industry.
The required material properties for producing ZnO films suitable as an efficient light emitter, as opposed to a TCO, are more stringent, which has hampered the development of ZnO light emitters. Specifically, the main issue has been the formation of undesirable native defects in ZnO, e.g. vacancies and interstitials of both Zinc and Oxygen atoms, which are deep-level defects that reduce the efficiency of emission at the bandgap energy by trapping the free excitons and substantially reducing the energy of any subsequent radiative emission, or favoring non-radiative emission, i.e. stored bandgap energy is lost to other undesirable pathways such as heat. Reducing (during process) and maintaining (post-process) the undesirable deep-level defect concentration to low values, while simultaneously providing (during process) an appropriate concentration of desirable shallow optical binding centers to prevent the free excitons from migrating to the deep-level defects, are the key elements needed to enable bandgap (or near bandgap) radiative recombination to dominate.
An object of the present invention is to overcome the shortcomings of the prior art by providing an electro-luminescent structure comprising a direct-bandgap semiconductor material, such as including zinc oxide, doped with exciton binding centers, while reducing the concentration of deep-level defects and controlling the concentration of shallow optical binding centers.